The invention relates generally to a catalyst composition and particularly to a catalyst composition and system for reducing nitrogen oxides (NOx) through selective catalytic reduction (SCR).
Exhaust streams generated by the combustion of fossil fuels in, for example, furnaces, ovens, and engines, contain nitrogen oxides (NOx) that are undesirable pollutants. There is a growing need to have efficient and robust emission treatment systems to treat the NOx emissions.
In selective catalytic reduction (SCR) using hydrocarbons (HC), hydrocarbons serve as the reductants for NOx conversion. Hydrocarbons employed for HC—SCR include relatively small molecules like methane, ethane, ethylene, propane, and propylene, as well as longer linear hydrocarbons like hexane, octane, etc., or branched hydrocarbons like iso-octane. The injection of several types of hydrocarbons has been explored in some heavy-duty diesel engines to supplement the HC in the exhaust stream. From an infrastructure point of view, it would be advantageous to employ an on-board diesel fuel as the hydrocarbon source for HC—SCR.
Fuels, including gasoline or diesel fuels containing sulfur lead to a number of disadvantages when trying to clean the exhaust gases by some form of catalytic after-treatment. During the combustion process, sulfur in the fuel gets converted to sulfur dioxide (SO.sub.2), which poisons some catalysts. Further poisoning happens from the formation of base metal sulfates from the components of a catalyst composition, which sulfates can act as a reservoir for poisoning sulfur species within the catalyst.
When the SCR catalysts absorb the NOx in the exhaust gas, they also absorb sulfur oxides (SOx) in the exhaust gas. The sulfur oxides poison the catalysts and as a result, the NOx absorption performance declines as the poisoning by the sulfur oxides (SOx) increases. Therefore, there is a need to reduce sulfur absorption by the SCR catalysts and prevent catalyst degradation.